Fix for iterators into unordered_maps

[kroening]

C++11 §23.2.5/13 states:

The insert and emplace members shall not affect the validity of references
to container elements, but may invalidate all iterators to the container.

This fixes two instances where the iterators are assumed to remain valid
while inserting additional elements into an unordered_map. The problem is
exposed when using -D_GLIBCXX_DEBUG.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6208 (c920010) into develop (0002950) will increase coverage by 7.99%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6208      +/-   ##
===========================================
+ Coverage    67.40%   75.40%   +7.99%     
===========================================
  Files         1157     1456     +299     
  Lines        95236   161217   +65981     
===========================================
+ Hits         64197   121566   +57369     
- Misses       31039    39651    +8612     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/util/string_container.cpp	52.94% <0.00%> (-47.06%)	⬇️
src/solvers/prop/prop.cpp	42.85% <0.00%> (-41.76%)	⬇️
src/solvers/flattening/boolbv_member.cpp	53.65% <0.00%> (-38.65%)	⬇️
src/cpp/cpp_storage_spec.cpp	65.00% <0.00%> (-35.00%)	⬇️
src/util/cmdline.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/array_pool.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/string_refinement.h	66.66% <0.00%> (-33.34%)	⬇️
...rs/strings/string_concatenation_builtin_function.h	0.00% <0.00%> (-33.34%)	⬇️
src/cbmc/c_test_input_generator.cpp	60.00% <0.00%> (-30.33%)	⬇️
src/analyses/local_cfg.h	75.00% <0.00%> (-25.00%)	⬇️
... and 1435 more

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[martin-cs]

PR is definitely an improvement. I thought there was a CI run with -D_GLIBCXX_DEBUG that should have caught this?

[kroening]

PR is definitely an improvement. I thought there was a CI run with -D_GLIBCXX_DEBUG that should have caught this?

It didn't -- perhaps this requires a more recent version of g++.